1. Field of the Invention
The present invention relates to a method of producing a magnetoresistive element used for a magnetic head, a potentiosensor, an angular sensor and the like.
2. Description of the Related Art
As an example of conventional magnetoresistive elements which employ a giant magnetoresistive effect, the magnetoresistive element disclosed in U.S. Pat. No. 5,206,590 is known.
As shown in FIG. 8, the magnetoresistive element A disclosed in this publication comprises a first ferromagnetic layer 2 made of a soft magnetic material, a non-magnetic layer 3, a second ferromagnetic layer 4 and an antiferromagnetic layer 5, which are laminated on a non-magnetic substrate 1. The magnetization direction B of the second ferromagnetic layer 4 is fixed by magnetic exchange coupling by the antiferromagnetic layer 5, and the magnetization direction C of the first ferromagnetic layer 2 is perpendicular to the magnetization direction B of the second ferromagnetic layer 4 in the absence of an applied magnetic field. However, the magnetization direction C of the first ferromagnetic layer 2 is not fixed and brought into a free state so that the direction C can be rotated by an external magnetic field.
When an applied magnetic field h is applied to the structure shown in FIG. 8, the magnetization direction C of the first ferromagnetic layer 2 is rotated as shown by arrows of chain lines in accordance with the direction of the applied magnetic field h, and an angle difference in magnetization direction occurs between the first ferromagnetic layer 2 and the second ferromagnetic layer 4, thereby causing a change in resistance. This enables detection of a magnetic field.
In manufacture of the magnetoresistive element A having the structure shown in FIG. 8, as a method of controlling the direction of the exchange coupling magnetic field produced by the antiferromagnetic layer 5 to an appropriate direction, there are methods including a method in which a magnetic field is applied in any desired direction (for example, the direction perpendicular to the easy magnetization axis of the first ferromagnetic layer 2 made of a soft magnetic material) in deposition of the antiferromagnetic layer 5, and a method in which after each of layers are laminated, the layers must be heat-treated by heating to a temperature over the blocking temperature and then cooling to room temperature while applying a magnetic field in the direction perpendicular to the easy magnetization axis of the first ferromagnetic layer 2 made of a soft magnetic material.
In the magnetoresistive element A having the above structure, the magnetoanisotropy of each of the magnetic layers is controlled by deposition in a magnetic field or heat treatment in a magnetic field, and thus the direction of easy axis of magnetization of the first ferromagnetic layer 2 is set to the same direction as the easy magnetization axis of the second ferromagnetic layer 4.
However, in a structure wherein the magnetization directions of the two ferromagnetic layers 2 and 4 are set to the same direction, the coercive force of the first ferromagnetic layer 2 which is free in magnetization rotation cannot be decreased, thereby consequently causing the problem of increasing the hysteresis in the minor loop of a magnetization curve obtained by the first ferromagnetic layer 2. Therefore, in the conventional structure shown in FIG. 8, it is necessary to employ a structure in which the rotation of magnetization of the first ferromagnetic layer 2 is stabilized by applying an excess of bias magnetic field